1. Field of the Invention
This invention relates to a current mirror circuit which is employable in an integrated circuit, etc. and more particularly to a current source circuit that is actuated by a low voltage.
2. Description of the Prior Art
A conventional example of a current mirror circuit is shown in FIG. 1. NPN transistors 1 and 2 are constituting a differential amplification circuit. A transistor 3 is a PNP transistor on the current input side. A transistor 4 is a PNP transistor on the current output side. A capacitor C.sub.1 and a resistor R.sub.B are stabilized AC feedback elements. A current source I.sub.IN is an input constant current source. A current source I.sub.OUT is an output constant current source. A current source I.sub.o is a constant current source and V.sub.cc is a power supply.
The minimum actuation power supply voltage for this circuit is roughly "V.sub.BE +2V.sub.CE " (where V.sub.BE is base-emitter voltage and V.sub.CE is collector-emitter voltage). When V.sub.BE =0.7 V and V.sub.CE =0.1 V, V.sub.BE +2V.sub.CE =0.9 V, and the circuit thus operates at low voltage. Also, thanks to the use of a feedback circuit, there is stable production of output current even if fluctuations in the h.sub.fe of lateral PNP transistors 3 and 4 occur.
However, in the conventional circuit of FIG. 1, it is necessary to have an AC feedback circuit C.sub.1 and R.sub.B with a large CR time constant, since the circuit operates as a differential amplification circuit and as an emitter-grounded amplifier for PNP transistor 3. That is, since the differential amplification circuit's load is between the base and emitter of transistor 4, the output voltage (AC feedback voltage) V1 is small and stability with respect to oscillation, etc. is poor. Improvement in this respect could be made by making the values of capacitor C.sub.1 and resistor R.sub.B large but from consideration of NPN transistor h.sub.fe fluctuation it is not possible to make the value of R.sub.B large. It is therefore necessary to increase C in order to increase the amount (to effect stabilization) of AC feedback. Although a feedback circuit is included, the circuit is unstable (is liable to oscillate), since the negative feedback phase and amplitude change due to the effects of parasitic capacitance and the f.sub.T (operating frequency at which h.sub.fe is 1) of PNP transistor 3.